City of Ember has almost anything one could want from a science fiction-based family adventure film: likeable characters, an imaginative setting, and a fast pace. The third item on that list is notable. Recognizing that children often have limited attention spans, the filmmakers have endeavored to keep City of Ember moving. Exposition, when needed, is delivered briskly and with a minimum of irrelevant information. Dialogue rarely accomplishes anything beyond advancing the plot. The almost complete absence of transition scenes results in a flow that is occasionally choppy. Overall, however, City of Ember will find favor with older children and their parents.

The film takes place more than 200 years in the future. Ember is an underground city that was developed to provide sanctuary for the last of the human race after a catastrophe wiped out the inhabitants of the surface of the planet. Now, after two centuries, Ember's infrastructure is beginning to fail. The generator that provides light, heat, and air to the city is experiencing frequent breakdowns. The water supply pipelines are springing leaks. The huge food storehouses are almost barren. And morale is at an all-time low.

Two children - Lina Mayfleet (Saoirse Ronan) and Doon Harrow (Harry Treadway) - have become Ember's self-appointed saviors. Lina is a messenger and Doon is a pipe repairman, and both recognize that the city is in jeopardy. Following cryptic clues, Lina and Doon determine that the path to salvation leads to the outside world. But it's not as simple as opening a secret door. The builders of Ember didn't want the inhabitants leaving before an appointed time, so they safeguarded the route to the surface. It's up to Lina and Doon to unravel the puzzle. The closer they get to the truth, however, the more dangerous they become to the city's corrupt mayor (Bill Murray), who determines they must be arrested lest they endanger a personal nest-egg he is feathering.

Aspects of City of Ember make it seem like a futuristic version of National Treasure where the prize is the survival of the human race. The things that made the Nicolas Cage adventure so popular - finding clues and decoding puzzles - are in evidence here, although the running around doesn't seem quite as repetitious and pointless. The climactic 15 minutes involve a lot of action sequences, and there are times when it almost seems as if they have been designed with a future amusement park ride in mind.

Director Gil Kenan's (Monster House) vision of Ember is inventive. The city is a cramped, claustrophobic place that owes a little to George Orwell and a little more to Terry Gilliam. The technology is a mix of pre-computer '50s and '60s engineering, with gears and motors dominating. The color palette is dominated by browns and other dull colors as a way of emphasizing how dreary life has become within what was once a technological marvel. There are also times when Ember feels like some kind of gigantic Rube Goldberg contraption, where it takes an exceedingly complicated number of steps to arrive at a relatively straightforward objective.

The film's stars are a pair of lesser-known actors whose careers are in ascent. Saoirse Ronan, by virtue of her Best Supporting Actress nomination for Atonement, is the higher profile of the two. She exhibits command of both her character and her American accent (despite being born in New York City, she was raised in Ireland), and imbues Lina with an optimism and energy that are infectious. Harry Treadway is not yet well known in the United States, but he has the looks and poise for that to change. The supporting cast is dotted with familiar names: Bill Murray as Ember's mayor; Tim Robbins as Doon's father; Martin Landau as Doon's mentor; and Mary Kay Place as a woman who takes in Lina and her young sister when their grandmother dies.

Although City of Ember has been crafted with a family audience in mind, there are a few scenes that might be deemed too frightening for smaller children. The underground passages of Ember are home to a giant mole-like creature and scenes of it chasing the protagonists could be nightmare fodder for younger, more sensitive boys and girls. Other than that, however, there's nothing objectionable about the film's content. And, unlike many post-apocalyptic stories, this one is built upon a foundation of hope, not despair. Those looking for thought-provoking material won't find it here but, as a well-paced adventure with sci-fi overtones, it's a good excuse to spend 90 minutes in a multiplex auditorium.